Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:1.12.7.2 (hydrogenase)
3,522 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The reduction of CO2 or any other methanogenic substrate to methane serves the same function as the reduction of oxygen, nitrate or sulfate to more reduced products. These exergonic reactions are coupled to the production of usable energy generated through a charge separation and a protonmotive-force-driven ATPase. For the understanding of how methanogens derive energy from C-1 unit reduction one must study the biochemistry of the chemical reactions involved and how these are coupled to the production of a charge separation and subsequent electron transport phosphorylation. Data on methanogenesis by a variety of organisms indicates ubiquitous use of CH3-S-CoM as the final electron acceptor in the production of methane through the methyl CoM reductase and of 5-deazaflavin as a primary source of reducing equivalents. Three known enzymes serve as catalysts in the production of reduced 5-deazaflavin: hydrogenase, formate dehydrogenase and CO dehydrogenase. All three are potential candidates for proton pumps. In the organisms that must oxidize some of their substrate to obtain electrons for the reduction of another portion of the substrate to methane (e.g., those using formate, methanol or acetate), the latter two enzymes may operate in the oxidizing direction. CO2 is the most frequent substrate for methanogenesis but is the only substrate that obligately requires the presence of H2 and hydrogenase. Growth on methanol requires a B12-containing methanol-CoM methyl transferase and does not necessarily need any other methanogenic enzymes besides the methyl-CoM reductase system when hydrogenase is present. When bacteria grow on methanol alone it is not yet clear if they get their reducing equivalents from a reversal of methanogenic enzymes, thus oxidizing methyl groups to CO2. An alternative (since these and acetate-catabolizing methanogens possess cytochrome b) is electron transport and possible proton pumping via a cytochrome-containing electron transport chain. Several of the actual components of the methanogenic pathway from CO2 have been characterized. Methanofuran is apparently the first carbon-carrying cofactor in the pathway, forming carboxy-methanofuran. Formyl-FAF or formyl-methanopterin (YFC, a very rapidly labelled compound during 14C pulse labeling) has been implicated as an obligate intermediate in methanogenesis, since methanopterin or FAF is an essential component of the carbon dioxide reducing factor in dialyzed extract methanogenesis. FAF also carries the carbon at the methylene and methyl oxidation levels.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:The bioenergetics of methanogenesis. 623 47

Maximum growth of Campylobacter fetus subsp. jejuni, strain C-61, occurred when the cultures were incubated with shaking in atmospheres containing approximately 30% hydrogen, 5% oxygen, and 10% CO2. Suspensions of cells grown under these conditions consumed oxygen with formate as the substrate in the presence of 0.33 mM cyanide, which completely inhibited respiration with ascorbate-N,N,N',N'-tetramethyl-p-phenylenediamine and with lactate. Spectroscopic evidence with intact cells suggested that a form of cytochrome c, reducible with formate but not with lactate or ascorbate-N,N,N',N'-tetramethyl-p-phenylenediamine, can be reoxidized by a cyanide-insensitive system. Analysis of membranes from the cells showed high- and low-potential forms of cytochrome c, cytochrome b, and various enzymes, including hydrogenase, formate dehydrogenase, and fumarate reductase. The predominant carbon monoxide-binding pigment appeared to be a form of cytochrome c, but the spectra also showed evidence of cytochrome o. The membrane cytochromes were reduced by hydrogen in the presence of 2-heptyl-4-hydroxyquinoline-N-oxide at concentrations which prevented the reduction of cytochrome c with succinate as the electron donor. Reoxidation of the substrate-reduced cytochromes by oxygen was apparently mediated by cyanide-sensitive and cyanide-insensitive systems. The membranes also had hydrogen-fumarate oxidoreductase activity mediated by cytochrome b. We conclude that C. fetus jejuni has high- and low-potential forms of cytochrome which are associated with a complex terminal oxidase system.
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PMID:Aerobic and anaerobic respiratory systems in Campylobacter fetus subsp. jejuni grown in atmospheres containing hydrogen. 628 61

Mutants of Escherichia coli were isolated in which transcription of the structural genes for hydrogenase (hyd) and for one of the components of formate dehydrogenase (fdh) (of the formate hydrogen-lyase complex) is coupled with that of the lacZ gene. They were--together with lac fusions of the nifH and nifL genes from Klebsiella--used to study regulation by redox control, of the expression of the respective structural genes. The following results were obtained: (i) beta-galactosidase synthesis was fully repressed in the presence of O2 or nitrate (anaerobically), and induced in the absence of an external electron acceptor. Fumarate as terminal electron acceptor only marginally affected nif expression and partially repressed hyd and fdh expression. Redox control of the synthesis of hydrogenase and formate dehydrogenase, therefore, (as well as that of nif) acts at the level of transcription; the size of the redox potential seems to be correlated with the amount of repression; (ii) beta-galactosidase synthesis in the hyd:: lac and fdh::lac fusion strains is induced by formate. At high concentrations formate reverses the repression by nitrate and fumarate but not that by oxygen.
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PMID:On the redox control of synthesis of anaerobically induced enzymes in enterobacteriaceae. 636 66

Citrobacter freundii 62 can grow in the absence of oxygen in media containing glucose, peptone, fumarate or malate. When the medium contained fumarate or malate, the culture could grow under anaerobic conditions only in the presence of molecular hydrogen, formate or nitrate. The highest activity of formatehydrogenlyase and hydrogenase was found when C. freundii grew in a medium with glucose and formate. The activity was lower in media with other organic substrates, particularly, in the absence of formate or H2. The activity of hydrogenase was very low in cells grown under aerobic conditions or in the presence of nitrates while the activity of formatehydrogenlyase was not found at all for all practical purposes. The activity of formate dehydrogenase assessed in the presence of methylene blue was rather high irrespective of the conditions under which the culture was grown. However, when the activity of formate dehydrogenase was determined in the presence of benzyl viologen, it was high only in cells grown in the medium with glucose and formate.
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PMID:[Enzyme activity of the formate hydrogenlyase complex in Citrobacter freundii]. 637 26

Carbon monoxide dehydrogenase from Clostridium thermoaceticum has been purified to homogeneity using a strict anaerobic procedure. The enzyme has a molecular weight of about 440,000 and it consists of three each of two different subunits giving the composition alpha 3 beta 3. The molecular weight of the alpha-subunit is 78,000 and that of the beta-subunit is 71,000. Pore limit gel electrophoresis gave a molecular weight of 161,000 indicating that the enzyme dissociates to form a dimer with an alpha beta structure. The dimer apparently contains per mol 2 nickel, 1 zinc, 11 iron, and 14 acid-labile sulfur. The anaerobic enzyme has an iron-sulfur type spectrum, which is changed in the presence of the substrate, CO. In the presence of oxygen, which destroys the activity or CO2, the spectrum is that of a typical iron-sulfur protein. Under acidic conditions a low molecular weight nickel factor separates from the enzyme. Viologens, methylene blue, ferredoxin, flavodoxin, and rubredoxin serve as electron acceptors. Of these rubredoxin is by far the most efficient. The enzyme has a pH optimum around 8.4. At this pH and 50 degrees C under 100% CO atmosphere, the apparent Km for methyl viologen is 3.03 mM and Vmax is 750 mumols of CO oxidized min-1 mg-1. Cyanide and methyl iodide inhibit the enzyme. CO reverses the cyanide inhibition but promotes the reaction with methyl iodide. The pure enzyme has no hydrogenase or formate dehydrogenase activity.
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PMID:Properties of purified carbon monoxide dehydrogenase from Clostridium thermoaceticum, a nickel, iron-sulfur protein. 668 89

A minimally defined medium was developed for the cultivation of the acetogen Clostridium thermoaceticum. The medium contained glucose as the carbon and energy source, ammonium sulfate as the nitrogen source, nicotinic acid as the sole essential vitamin, reductant, a phosphate-bicarbonate buffer, mineral salts and chelator, and a CO2 gas phase. Adaptation of C. thermoaceticum from undefined medium containing yeast extract and tryptone to the minimally defined medium required sequential passage on defined medium supplemented with amino acids and vitamins. Growth and cell yields were reduced on the minimal medium, but the activities of carbon monoxide dehydrogenase, hydrogenase, and formate dehydrogenase were comparable between undefined and minimal media.
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PMID:Development of a minimally defined medium for the acetogen Clostridium thermoaceticum. 674 75

The acetogenic bacterium Clostridium thermoautotrophicum, grown on methanol, glucose, or CO2-H2, contained high levels of corrinoids, formate dehydrogenase, tetrahydrofolate enzymes, carbon monoxide dehydrogenase, and hydrogenase. Cell-free extracts catalyzed pyruvate-dependent formation of acetate from methyltetrahydrofolate. These results suggest that C. thermoautotrophicum synthesizes acetate from CO2 via a formate-tetrahydrofolate-corrinoid pathway.
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PMID:Levels of enzymes involved in the synthesis of acetate from CO2 in Clostridium thermoautotrophicum. 680 50

A soluble yellow CO dehydrogenase from CO-autotrophically grown cells of Pseudomonas carboxydohydrogena was purified 35-fold in seven steps to better than 95% homogeneity with a yield of 30%. The final specific activity was 180 mumol of acceptor reduced per min per mg of protein as determined by an assay based on the CO-dependent reduction of thionin. Methyl viologen, nicotinamide adenine dinucleotide (phosphate), flavin mononucleotide, and flavin adenine dinucleotide were not reduced by the enzyme, but methylene blue, thionin, and toluylene blue were reduced. The molecular weight of native enzyme was determined to be 4 x 10(5). Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate revealed at least three nonidentical subunits of molecular weights 14,000 (alpha), 28,000 (beta), and 85,000 (gamma). The ratio of densities of each subunit after electrophoresis was about 1:2:6 (alpha/beta/gamma), suggesting an alpha(3)beta(3)gamma(3) structure for the enzyme. The purified enzyme was free of formate dehydrogenase and nicotinamide adenine dinucleotide-specific hydrogenase activities, but contained particulate hydrogenase-like activity with thionin as electron acceptor. Known metalchelating agents tested had no effect on CO dehydrogenase activity. No divalent cations tested stimulated enzyme activity. The native enzyme does not contain Ni since cells assimilated little (63)Ni during growth, and the specific (63)Ni content of the enzyme declined during purification. The isoelectric point of the native enzyme was found to be 4.5 to 4.7. The K(m) for CO was found to be 63 muM. The spectrum of the enzyme and its protein-free extract revealed that it contains bound flavin. The cofactor was flavin adenine dinucleotide based on enzyme digestion and thin-layer chromatography. One mole of native enzyme contains at least 3 mol of noncovalently bound flavin adenine dinucleotide.
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PMID:Purification and some properties of carbon monoxide dehydrogenase from Pseudomonas carboxydohydrogena. 689 15

Proton translocation by Desulfovibrio desulfuricans cells, cultured anaerobically with nitrate as terminal oxidant, was studied by the oxidant-pulse method. Nitrate-grown D. desulfuricans translocated protons rapidly and reproducibly with hydrogen as reductant and nitrite as electron acceptor. H+/2e- ratios were typically in the range 1.8-2.2. Proton translocation following pulses of nitrite was also observed with endogenous substrate in freshly harvested cells and with lactate or formate as electron donors in starved cells. Problems in the determination of H+/2e- ratios when endogenous substrate, formate, or lactate was the electron donor are discussed. Evidence is presented for the location of formate dehydrogenase, hydrogenase, and nitrite reductase on the periplasmic and for lactate dehydrogenase on the cytoplasmic side of the cytoplasmic membrane.
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PMID:Proton translocation associated with nitrite respiration in Desulfovibrio desulfuricans. 701 54

Six fdn mutants of Salmonella typhimurium defective in the formation of nitrate reductase-linked formate dehydrogenase (FDHN) but capable of producing both the hydrogenase-linked formate dehydrogenase (FDHH) and nitrate reductase were characterized. Results of phage P22 transduction experiments indicated that there may be three fdn genes located on the metE-metB chromosomal segment and distinct from all previously identified fdh and chl loci. All six FDHH+ FDHN- mutants were found to make FDHN enzyme protein which was indistinguishable from that of the wild type in electrophoretic studies. However, the results of the spectral studies indicated that all six mutants were defective in the anaerobic cytochrome b559 associated with FDHN. All contained the cytochrome b559 associated with nitrate reductase in amounts equal to or greater than the wild type. The results of the transduction experiments also indicated that the metE- metB segment of the Salmonella chromosome resembles that of Escherichia coli more than was originally thought.
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PMID:Salmonella typhimurium mutants defective in the formate dehydrogenase linked to nitrate reductase. 703 33


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